Three well known methods exist for producing a gloss surface on a waxed paper surface. It has long been known to use the so-called "dip" method for producing a gloss surface on containers whereby a previously-waxed cup having a coating of hot liquid wax is immersed or "rolled" through a liquid coolant such as water to create the gloss appearance. Although this early technique could be used successfully to create a gloss surface, it has a number of disadvantages. For example, the gloss using the dip method tends to be uneven and exhibits a "rippled" visual appearance. The problem becomes more severe if the wax coating itself is non-uniform, i.e., heavier on certain areas of the cup surface than others due to a non-uniform application in the wax treater or because of temperature variations of the melted wax before cooling on the cup surface.
Even if the liquid wax coating is applied in a smooth and uniform manner, the dip technique tends to cause a non-uniform cooling/crystallization of the wax due to the action of the coolant as it contacts the melted wax. One probable explanation for such non-uniformity is that the water (or other cooling medium) disturbs the surface of the liquid wax upon contact with the cup surface, thereby resulting in the uneven "rippled" appearance. In addition, any such surface imperfections tend to distort the appearance of graphics on the cup such as the design artwork or printing, making the end product commercially unacceptable.
The formation of a smooth high gloss surface on containers is made more difficult because paraffin-type waxes used on conventional cups have varying molecular weight distributions. As a general proposition, during cooling the higher fractions initially form tiny surface crystals (creating the gloss appearance) while forming a matrix to hold the remaining uncongealed liquid wax. Thus, it is important that the wax be rapidly and uniformly cooled along the entire cup surface at the same time in order to obtain a uniform formation of surface crystals. As indicated above, it is also essential that the yet unhardened wax remain undisturbed during the cooling step.
Thus, one additional problem with the conventional "dip" method is that it requires that the cooling take place within a very narrow and controlled temperature range for the congealing liquid wax (typically in the range of only 3-5 degrees Fahrenheit) in order to obtain a uniform gloss. As a result, the "dip" method poses a very significant quality control problem in any commercial application because of the narrow effective temperature range and resultant lack of operating flexibility.
A second known method for cooling previously waxed cups to create a gloss surface uses one or more streams of cooling air. Again, it is very difficult to obtain a uniform gloss surface using such techniques primarily because of the problems in maintaining a constant wax temperature during cooling along the side walls of the cup and because the low thermal conductivity of air requires a velocity and flow volume which results in disruption of the liquid wax surface in its uncongealed state more severe than that encountered with the dip method.
A third known method for producing a surface gloss uses a water spray technique which likewise cools the liquid wax as it passes through a cooling chamber. Again, however, the prior art spray techniques tend to cause surface imperfections due to the physical impact of spray droplets against the film of congealing liquid wax.
As an alternative to the traditional wax coating/cooling techniques for creating a gloss surface on paper products, cup manufacturers more recently have begun to use plastic coatings (such as polyethylene or polystyrene) to produce a uniform gloss or "glaze"-like surface on the product. Typically, such coatings are applied to both exterior and interior container surfaces. Although the so-called "double-sided poly" cups have improved the consistency and uniformity of gloss products, they have certain distinct disadvantages. For example, the plastic coating on the cups is essentially non-biodegradable and therefore presents environmental concerns as non-disposable wastes. In addition, unlike wax-coated paper products, the "double-sided poly" products cannot be recycled. They also tend to have reduced sidewall stiffness as compared to wax cups or may leak because of deficiencies in the forming and sealing process.
It has now been found that it is possible to produce a high gloss on previously waxed paper sheet stock or containers while avoiding the above problems of appearance and uniformity. It has also been found that a commercially viable replacement product for double-sided poly coated cups can be produced using a process for treating previously coated wax cups which results in a surface having equal or better characteristics of gloss, stiffness and graphic appearance.
In one preferred exemplary embodiment, the process according to the present invention uses a thin uniform film of coolant such as water which impacts a rotating cup at a specific angle of orientation when the cup is moving at a constant linear speed. The film of water "touches" (but does not disturb) the uncongealed liquid wax coating on the cup. The water is applied to the cup at a minimum (preferably zero) relative velocity, i.e., minimum relative to the rotational (angular) velocity of the cup itself, and at a position parallel to the cup side wall and tangent to the outer diameter of the cup.
In particular, it has now been found that a uniform high gloss surface can be obtained by (1) applying a substantially uniform coating of melted wax material to the outside surface of the cup; (2) maintaining the coated wax surface at a uniform temperature above the wax melting point (and therefore above the congealing temperature); (3) rotating the cup at a uniform spin rate, thereby imparting a constant angular velocity to the cup; and (4) applying a thin, substantially uniform film of cooling medium (preferably water) to the cup tangential to the rotating cup surface under laminar flow conditions, i.e., at a precisely controlled velocity, temperature and volume such that the water effectively "wraps" around the cup with minimum disturbance to the uncongealed wax. The laminar flow of water around the rotating cup actually causes the water to be drawn onto the cup surface at it turns. In the preferred embodiment of the invention, the relative velocity between the rotating cup and the applied film of water on the surface is approximately zero.
In an alternative embodiment of the present invention, it has also been found that a uniform gloss surface can be produced on a previously coated and cooled wax cup, i.e., a cup having a congealed "satin" non-gloss wax coating, by (1) reheating the previously waxed cup to a temperature above the melting point of the wax coating; (2) maintaining the coated wax surface at a uniform temperature above the wax melting point; (3) rotating the cup at a uniform spin rate to thereby impart a constant angular velocity to the cup; and (4) applying a thin, uniform film of cooling medium to the cup tangential to the rotating cup surface whereby the relative velocity between the rotating cup and the applied film of water on the surface is approximately zero.
The same basic process steps according to the invention may also be used to create a high gloss surface on previously waxed flat webs or sheet stock. Again, the relative velocity of the flat coated wax surface and the cooling water film is at a minimum, preferably zero at the point of contact with the water.
In one exemplary embodiment, the process according to the invention maintains the wax temperature on the cup surface before cooling/gloss formation at approximately 160.degree. F. with a cup spin rate of approximately 285 rpm. The preferred angular cup velocity falls in the range of 250-400 rpm depending on cup diameter. The water temperature is normally held at about 45.degree. F., and the "narrow cut" paraffin wax used as the coating has an average melting point of 140.degree. F., preferably in the range of 130.degree. F. to 140.degree. F.
Significantly, it has also been found that the available temperature "window" for applying the cooling/gloss step in accordance with the present invention may be as much as 40.degree. F. above the wax melting point (rather than the 3 to 5 degree range available using conventional methods), depending on the wax composition. Because the cooling on the cup surface takes place without otherwise disturbing the liquid wax surface, the process offers a significantly greater degree of operating flexibility.
The process of cooling a cup with a film of water in accordance with the invention results in an average gloss surface reading of 68 as compared to double-sided poly cups which normally have a gloss surface reading of approximately 60 for readings taken on unprinted flat stock using a standard Photovolt Model 670 Reflection Meter.
It has also been found that the present method for producing a gloss surface on paper containers or sheet stock results in a product which is both bio-degradable and capable of being recycled. The underlying graphics are also equal to or better than those produced when using poly paper.
Other advantages of the invention include (1) the improved sealing capability of the wax cup as compared to double-sided poly cups; (2) improved sidewall stiffness (3) reduced cost of production (due to the use of a lower basis weight paper and a less costly coating material); and (4) high operating flexibility, i.e., commercially acceptable products produced within relatively broad ranges of operating conditions for the wax coating and cooling water temperatures.
Thus, it is an object of the present invention to produce a uniform high gloss on a previously wax-coated cup or sheet stock comparable to the presently available double-sided poly products, i.e., having equal or better characteristics of gloss, stiffness and graphic appearance.
It is still a further object of the present invention to produce a paper container or sheet stock having a high gloss surface which is capable of being recycled and which offers improved sealing characteristics as compared to double-sided poly articles.
It is still a further object of the present invention to produce a high gloss wax-coated cup or sheet stock more efficiently and economically than conventional double-sided poly articles.
These and other objects of the present invention will become more clear upon a review of the following examples, appended drawings and description of the preferred exemplary embodiment.